1. Field of the Invention
The present invention generally relates to a distortion compensating device, and more particularly, to a distortion compensating device in a transmitter or a receiver which can compensate for nonlinear distortion, the nonlinear distortion being caused by a digital signal being distorted in the path from modulation with a matting in the transmitter to demodulation in the receiver.
2. Description of the Related Art
In a digital multiple radio communication, linear modulation such as phase-shift-keying (PSK) modulation and quadrature-amplitude modulation (QAM) is desired to be used for an effective use of frequency. In terrestrial radio communication, a 256QAM system is currently in practical use. In digital mobile communication systems in which a nonlinear modulation is applicable for portability of radio equipment, 4PSK modulation and 16QAM are currently used. To obtain good transmission performance by using such linear modulation systems, it is necessary to produce a high-power transmission signal.
However, when the power of the transmission signal is increased in conventional multiple communication equipment using linear modulation, a nonlinear range in a transmitting-linear amplifier is used. As a result, the transmission signal is distorted and the transmission performance is degraded. Maximum transmission power is determined by a tradeoff with the distortion. Therefore, to obtain a superior transmission performance, a high-efficiency and high-power linear amplifier is required.
On the other hand, in the receiver, to improve carrier to noise ratio (C/N) performance, an RF amplifier having a large gain is commonly used in the front end. However, a high-level signal is applied to the RF amplifier, the RF amplifier is saturated and the received signal is distorted. Therefore, even though the high-level signal is received, the received signal needs to be reduced by an attenuator, etc., and, thus, the high C/N obtained with the high level signal is degraded. Such attenuation of the high-level signal is performed because sufficient transmission performance for the system can be obtained even if the high-level signal is slightly attenuated. However, in the future, for large capacity data transmission, a much better transmutation performance is desired.
At present, several methods of compensating the distortion due to the nonlinear characteristics of the transmitting amplifier are proposed. FIG. 1 shows a block diagram of an example of a transmitter having a conventional distortion compensating function. The transmitter comprises a mapping-and-waveform-shaping circuit 10, a quadrature modulator 20, a transmission stage 30, and an antenna 40. The transmitter is constructed with a quadrature modulation technique, in which a baseband digital signal is divided into an I-channel signal and a Q-channel signal, and the two signals modulate 90.degree.-phase-shifted carrier signals to produce a modulated signal by combining the two modulated carrier signals. In the quadrature modulation technique, phase and amplitude of the baseband signal is mapped on a quadrature plane referred to a signal space diagram in which the I axis and the Q axis indicate the carrier signal.
In the mapping-and-waveform-shaping circuit 10, the baseband signals which are divided into the I-channel signal and the Q-channel signal are, respectively, converted to signals for being mapped on the signal space diagram constructed with the I axis and the Q axis, and modulating signals are produced through a roll-off filter. In the quadrature modulator 20, carrier signals Lo are respectively modulated by the I-channel and Q-channel signals, and are summed with each other to produce the modulated signal. In the transmission stage 30, the modulated signal is converted to a signal at the RF frequency, and is amplified by the transmitting amplifier to be transmitted through the antenna 40.
FIG. 2 shows a signal space diagram indicating a relationship between the baseband signal and the carrier signal in 16QAM. For example, in 16QAM, to map the I-channel and the Q-channel signals to one of 16 signal points D1 to D16 (hereinafter, referred to as specified signal points) arranged on the quadrature signal space diagram, the I-channel and Q-channel signals are respectively converted to signals, each having 4 levels. In general, by equalizing each intervals between the 4 levels, the 16 signal points are uniformly arranged on the signal space diagram.
In the receiver side, these signal points are discriminated by threshold lines (represented by dashed lines in FIG. 2) drawn among the signal points in the signal space diagram. The conventional threshold lines are drawn at the center between each two signal points taking into account noise which influences each signal point in the same manner. However, by the strong amplification in the transmission stage 30, the uniformly mapped signal points may be distorted, for example, as shown by signal points X.sub.1 to X.sub.4 in a first quadrant of FIG. 2. In a more specific case, some distorted signal points may fluctuate to another threshold area such as the signal point X.sub.3. As a result, the signal point X.sub.3 is discriminated to a wrong signal point as an error. To prevent such fault discrimination, there is a conventional mapping method in which opposite characteristics of the distortion characteristics of the amplifier in the transmission stage 30 are previously set with the mapping circuit 10. By using this method, at the output of the amplifier in the transmission stage 30, a signal point arrangement substantially similar to the specified signal point arrangement may be obtained.
FIG. 3 shows a block diagram of another example of a transmitter having a conventional distortion compensating function. The transmitter comprises a mapping-and-waveform-shaping circuit 11, a quadrature modulator 21, a transmission stage 31 and an antenna 41. Each circuit has the same function as that of the circuit in the transmitter shown in FIG. 1. In the transmitter, according to the shape of the baseband signal passed through the roll-off filter, an operational point of the transmitting amplifier in the transmission stage 31 is changed to extend the dynamic range of the transmitting amplifier. Since the carrier signal is modulated by the baseband signal passed through the roll-off filter, the level of the modulated signal is determined by the amplitude of the baseband signal from the roll-off filter. For example, the level of the modulated signal which is modulated by the baseband signal close to the origin in the signal space diagram is low, and the level of the modulated signal which is modulated by the baseband signal at the surrounding signal points is high. A level change of the modulated signal can be known before transmitting the modulated signal because the baseband signal to be transmitted is known. Therefore, by changing the operational point of the transmitting amplifier according to the level of the modulated signal, dynamic range of the amplifier may be increased.
However, the following problems occur in the above-mentioned conventional distortion compensating devices.
In the distortion-compensating device shown in FIG. 1, when the distortion characteristics due to the nonlinearity of the transmitting amplifier in the transmission stage 30 changes, the proper distortion compensation may not be performed with the previously-set opposite characteristics in the mapping-and-waveform-shaping circuit 10. The case where the distortion characteristics of the transmitting amplifier change may be due to a change of the input level applied to the transmitting amplifier, an exchange of the transmitting amplifier, and a temperature increase and aging of the transmitting amplifier. Therefore, the opposite characteristics need to be adapted to such changes. This adaption requires the hardware to be complex. More specifically, to deal with the exchange of the transmitting amplifier, the transmission stage 30 in the RF circuit and the mapping-and-waveform-shaping circuit in the baseband circuit need to be integrated. This may cause complexity of the configuration of the distortion compensating device.
In the distortion compensating device shown in FIG. 3, the waveform-shaping circuit needs to be constructed with a digital signal processing, and an amplifier whose operational point is controllable is required for the transmission stage 31. Therefore, the configuration of the distortion compensating device becomes complex.
Furthermore, in the distortion compensating devices shown in FIGS. 1 and 3, only distortion occurring in the transmitter side is compensated, the distortion occurring in the receiving amplifier in the receiver side cannot be compensated.